As a conventional technique, Patent Literature 1 listed below proposes a method in which three phase inverters according to a PWM modulation method are included, a plurality of fan motors are connected as loads of the respective inverters, a plurality of fan-motor control units as many as the fan motors to control the corresponding fan motors and a system control unit that controls the entire system are provided, and data communication is performed between the system control unit and the fan-motor control units at a time of controlling the fan motors, thereby controlling the fan motors.